A natural killer cell (NK cell) is a lymphoid cell playing a role in an immune reaction. Since the natural killer cell has a variety of functions and particularly has strong activity of killing a tumor cell, it is believed that the cell is an important member of immunological surveillance mechanism for removing abnormal cells which have become tumorigenic or are becoming tumorigenic in a living body. Therefore, study for effectively utilizing the cell in tumor therapy has been conducted for a long time.
For example, when a large amount of interleukin (IL)-2 which is one kind of lymphokines is added to a culture medium of a peripheral blood mononuclear cell (PBMC), so-called lymphokine activated killer lymphocytes (LAK cells) are proliferated in around one week in the case of a human. The LAK cells are well-known to comprise many NK cells. The LAK cells are widely used in adoptive immunotherapy of tumors. The LAK cells are also known to be effective in infectious diseases, and draw attention as strategy for treating infectious diseases which are difficult to cure with antibiotics.
Until now, a number of methods of ex vivo culturing NK cells have been reported (e.g. Non-Patent Literature 1), which are classified into a method with an accessory cell and a method without an accessory cell. The method without an accessory cell is a method comprising stimulating PBMC with IL-2 or IL-15. However, this method often results in a low cell expansion rate and a low NK cell content rate after culturing, and therefore, NK cells may not be obtained in an amount necessary for treatment depending on a donor of PBMC. Further, it has been reported that when a culture period is prolonged in order to obtain a large amount of NK cells, cytotoxic activity is reduced.
On the other hand, the method with an accessory cell is, for example, a method comprising using a K562 cell which is a B lymphoma cell strain, a Daudi cell which is a malignant lymphoma cell strain, or a HFWT cell which is a Wilms tumor cell strain, which has been subjected to radiation. Further, a method comprising using as an accessory cell the above-mentioned tumor cells into which a cytokine expressing gene is transferred has been also reported. However, since these accessory cells are tumor cells, allo cells and gene-transferred cells, much attention is necessary for securing safety from a viewpoint of injection into a living body of a NK cell mixed cultured with an accessory cell. In addition, a method comprising using PBMC irradiated with radiation as an accessory cell has been also reported (Patent Literature 1). However, this method necessitates a step of removing a CD3-positive cell as a cell population containing a NK cell, and further necessitates preparing a large amount of PBMC from a donor. A cell expansion rate by this method is maximally about 140-fold after 22 days from the initiation of culture.
A bioresponse modifying agent is a substance which regulates immune mechanism of a living body and is expected to have therapeutic effect on a variety of diseases, and is used in immunotherapy of a cancer. However, it is known that the agent does not activate lymphocyte function to such an extent that is expected. For this reason, such a treatment that the agent is directly administered to a patient requiring activation of lymphocyte function to activate the function is not generally performed, and is used only on limited cases under the current circumstances. The action mechanism of the bioresponse modifying agent does not comprise direct action on a tumor cell, and it is believed that the bioresponse modifying agent activates an immune cell inherent to a living body to which the agent is administered, and exerts anti-tumor effect via action of the immune cell. Therefore, the effect of administration of the bioresponse modifying agent is greatly influenced by the natural immunological state of a living body which has received the administration of the bioresponse modifying agent, and undergoes influence of a background factor of the living body. Thus, it is said that there are often differences of the anti-tumor effect between individuals and differences of action strength depending on the clinical states of patients.